Fossils discovered east of Africa's Lake Turkana confirm that there were two additional species of our genus—Homo—living alongside our direct human ancestral species, Homo erectus, almost two million years ago. The finds, announced in the journal Nature, include a face, a complete lower jaw, and part of a second lower jaw.

The fossils were uncovered between 2007 and 2009 by the Koobi Fora Research Project (KFRP), led by Meave and Louise Leakey. NYU anthropology professor Susan Antón, a member of the research team, was part of the effort to compare these fossils with those from earlier finds.

"These new fossils provide great tests of earlier hypotheses of how diverse the early Homo record was," explains Antón. "They provide anatomical support for the idea of multiple species of early Homo, but, more importantly, they suggest ideas about how the species might have divided up the environment—the species weren't separated into a large species and a small species; instead, the fossils suggest remarkable size variation within each species, but with different facial anatomies."

Four decades ago, the KFRP discovered the enigmatic fossil known as KNM-ER 1470 ("1470" for short). This skull, distinguished by its large brain size and long flat face, ignited a longstanding debate about just how many different species of early Homo lived alongside Homo erectus during the Pleistocene epoch, which spanned from 2.6 million to 11,700 years ago. 1470's unusual morphology was attributed by some scientists to sexual differences and natural degrees of variation within a single species, whereas others interpreted the fossil as evidence of a separate species.

This decades-old dilemma has endured for two reasons. First, comparisons with other fossils have been limited due to the fact that 1470's remains do not include its teeth or lower jaw. Second, no other fossil skull has mirrored 1470's flat and long face, leaving in doubt just how representative these characteristics are. The new fossils address both issues.

"For the past 40 years we have looked long and hard in the vast expanse of sediments around Lake Turkana for fossils that confirm the unique features of 1470's face and show us what its teeth and lower jaw would have looked like," says Meave Leakey, co-leader of the KFRP and a National Geographic Explorer-in-Residence. "At last we have some answers."

"Combined, the three new fossils give a much clearer picture of what 1470 looked like," adds Fred Spoor, leader of the scientific analyses. "As a result, it is now clear that two species of early Homo lived alongside Homo erectus. The new fossils will greatly help in unraveling how our branch of human evolution first emerged and flourished almost two million years ago."

Found within a radius of just over 10km from 1470's location, the three new fossils are dated between 1.78 million and 1.95 million years old. The face KNM-ER 62000, discovered in 2008, is very similar to that of 1470. Moreover, the face's well-preserved upper jaw has almost all of its cheek teeth still in place, which for the first time makes it possible to infer the type of lower jaw that would have fitted 1470. A particularly good match can be found in the other two new fossils, the lower jaw KNM-ER 60000, found in 2009, and part of another lower jaw, KNM-ER 62003, found in 2007. KNM-ER 60000 stands out as the most complete lower jaw of an early member of the genus Homo yet discovered.

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5 comments

Where do you draw the line between changes and a species? Germanics have a clearly distinctly different skull shape to Africans, but you wouldn't call them separate species - we have no trouble breeding, after all. At what point, going backwards, do facial changes jump into being a separate species? I believe heidelbergensis is the first step back, but aren't changes individual and gradiated? Or do significant changes happen suddenly, that we can distinguish between "modern human" and "the last Homo species"? Could someone explain this to me, please? My understanding of human evolution is pretty weak. Thanks

The definition of a "species" is arbitrary and has changed many times over last hundred years or so. Currently two organisms are considered of the same species if they can mate and produce fertile offspring. The definition that we use reflects that which is important to us at the time, and the current definition suits us well given our current understanding of evolution.

Most people's problem with evolution is that they see it as a series of discrete steps where a species remains unchanged for a long time and then suddenly becomes something significantly different... this generally does not happen. In reality it is more a continuous transition with branching and dead ends. This does not mean that rate of significant change is continuous, it just means that change is continuous. The birth of every new organism presents a new form to be accepted or rejected by the forces of natural selection.

The fact that some changes seem more significant TO US does not mean that they are any more significant at the genetic level where they occur than other changes that are less significant to us. A genetic change might cause a completely unnoticeable change in the phenotype of the organism, where another genetic change of no more complexity than the first can significantly change the appearance of the organism. It is possible for large changes to occur in the phenotype expression of a population rapidly, but this is generally in response to significant and rapid changes to the environment causing a significant change to selection pressures on the population. However, the rate of genotype change is almost constant, mutations occur with some degree of regularity, and inheritance/recombination occur with the birth of EVERY organism.

My point is, while it may appear that organisms change suddenly the underlying mechanism has a much more constant rate.

If you wanted to build a castle with Lego's but you could only use the Lego's that I handed to you and I handed you one new random Lego piece every few seconds and you could choose to use it or discard it then you would gradually start to form a castle... but if you suddenly changed your mind and wanted to build a race car instead then there would be a rather sudden change in the types of Lego bricks that you chose to use and the types that you decided to throw out and the model would change rather rapidly at first.

Me handing you a lego brick at a constant rate represents mutations and recombination, these are fairly constant processes. You choosing to use the piece I hand to you or not represents natural selection, and changing your mind regarding what model you're building represents a sudden environmental change that significantly affects the selection pressures acting on the population. The underlying mechanism is constant, but the observable change can be rapid.

Thanks for the explanation. Would it be true to add that some lego bricks may be attached that I no longer want, will not necessarily be removed or that I was given some bricks and just attached them to see if it works, but if there is no detrimental effect, the bricks may well end up there for a very long time?

I always thought it to be a gradual process, with no "major changes at each point". I get the idea two different species, evolving one from the other, is more like a Venn diagram consisting of lines. At one end, it is the simple organism, as it evolved, it became different, but not significant enough that if the older varieties still existed, they couldn't reproduce, but as the evolutionary venn moved further to the newer end, it became genetically significantly different that it wouldn't have enough in common with the oldest varieties to share the traits to reproduce.

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